Thermionic discharge device



A. w. HULL 2,246,176

THERMIONIC DISCHARGE DEVICE Filed April 8, 1959 June 17, 1941.

Fig. l. nfl-2.2.

Inventor: Alberi-- W. HUH, afwy. MLM

His Attofny.

Patented June 17, 1941 UNITED' STATES` PATENT oFFlcE 'i 2,246,116 f l THEBMIONIC DISCHARGE DEVICE New York Application April 8, 1939, Serial No. 266,803 4 claims. (ci. 25o-27.5)

The .present invention relates to electrical devices, such as rectiflers, oscillators, amplifiers,

and other devices in which a member is heated lower than would be required in the absence of such coating. The gradual loss of coating material by disintegration and evaporation eventually renders such cathodes inactive, that is, of W electron emissiv-ity. Various alkali and alkaline earth metals in the vapor state also have been proposed for enhancing the electron emission of heated thermionically emitting cathodes. such metals are highly reactive, and as their vapors cannot readily be coni-ined,` the application of such vapors to heated cathodes entails various difficulties which have limited their use.

In accordance with my present invention I have provided improved thermionic cathodesy which are provided with means for enhancing electron emission which operate on a new principle, and which are operable over longer periods of useful life than thermionic cathodes heretofore in use.

In a cathode embodying my present invention a material, the vapor pressure of which at the temperature of the cathode is imperceptible (less than 10- mm.) is rendered available for the activation of the cathode surface by a continuously acting. dispenser. The latter comprises a reagent from which the activating material is progressively generated and slowly vaporized during the life of the cathode. The dispenser preferably is constructed to provide restricted, pore-like migration paths whereby the activating material reaches the exterior of the dispenser from whence it is slowly CII side elevation, partly in section, of an electron discharge device embodying my present invention; Figs. 2, 3 and 3a are fragmental views of modified cathode structures; Fig. 4 is an enlarged fragmental view of a cathode emitting surface structure; Figs. 5 and 6 respectively are longitudinal section and end views of another modification; and Fig. 7 is a fragmental sectional view of arnodii'lcation.

'I'he device shown in Fig. 1 comprises an elongated sealed envelope I of glass, in the opposite ends of which an anode 2 and a cathode are respectively provided. The anode is carried by a conductor 4 which is sealed into` a glass stem 5. The cathode 3, with which my invention is concerned, presently will be described in detail. It is surrounded by a heat shield 6 consisting of multiple closely spaced walls of metal. The heat shield may assume various forms, and may consist of refractory,non-conducting material. The shield cover lcontains orifices, one of which is indicated at 0, for the passage of the discharge between the electrodes 2 and 3. A single orice may be provided. Surrounding -the discharge space is a discharge-conning metal shield 8 which is supported by the wires 9, I0. These wires are held by the glass stem II, being attached thereto by the collar I2, and are insulated and spaced from the heat shield by insulators I3, I3. I'he extremity of the shield 8 adjacent to the anode is anchored to a tubular glass extension I4 of the stem 5 by a collar I5. A perforatedgrid I6 of graphite or other suitable material is mounted Within the shield 8 by screws, as indicated, for the control of the space current between the electrodes. 'Ihe grid I6, of course, is

omitted Whenthe discharge tube is to be used solely for rectification purposes.

The cathode 3 comprises a plurality of strips or sheets, or mesh, or combination of. theseherein termed ribbons, and consisting for example of and continuously distilled to theI electronthin molybdenum. The cathode members lin this case are radially spaced about a rod-shaped, elongated dispenser I8, although such arrangement is not an essential feature of the present invention. 'I'he dispenser may consist of a closely woven Wire-mesh tube which contains a charge which when heated is capable of yielding a material for enhancing theelectron emissivity of the cathode, as will be described. Another form of dispenser is. shown in Fig. 5. The dispenser I8, Fig. 1, may be heated in any convenient way, as by passage of current, to an elevated temperature which ordinarily should be higher than the operating temperature ofthe cathode.

One terminal of vthe dispenser, which also functions as a cathode heater, is connected as by binding with fine .-wire I9, to a sealed-inconductorA 23 which'is' insulated from the cathode andthe 'heatilhield by an insulator 2|, consisting of magnesia, alumina, or other suitable refractory material. The other terminal of the dispenser in the structure of Fig. 1 is connected to the cathode, the circuit being completed by a sealed-in conductor 22 which is insulated from the heat shield by an insulator 23. A conductor 24, insulated from the shield by an insulator 25, supplements the conductor 22, and may be connected thereto for carrying load current to the cathode. A sealed-in conductor 26 leads to the grid, through the intermediary of the support wire' 3.

The heat shield in the device illustrated is connected to the cathode conductor 22 by a conductor 21 containing a resistor 28 of high resistance value (one thousand to one million ohms), but this resistance connection may be omitted. 'I'he heat shield may be left electrically floating or unconnected tothe cathode. On the other hand, in some cases the shield may be connected to the cathode through a low resistance circuit, the resistor 28 being omitted.

The upper end of the cathode and dispenser may be mechanically supported by the heat shield, as indicated, an insulator 30, 3l being provided to electrically insulate the cathode and dispenser from the heat shield.

The envelope contains a charge of gas or vapor at low pressure which ordinarily will be at a pressure within the range of a few microns to about one or a few millimeters of mercury. For example, the gas filling may consist of a rare gas or of mercury vapor at a pressure of 5 to 25 microns. A quantity of mercury, indicated at 33 near the cathode stem, serves as a source of vapor.

As best shown in Figs. 2 and-3a, the tubular dispenser contains a charge which is slowly reactive at high temperatures to yield vaporizable material which is capable of enhancing electron emission. The dispenser may contain a quantity of oxide of alkaline earth metal, such as barium oxide or strontium oxide, or a mixture of such oxides. The alkaline earth oxide may be mixed with, and preferably fused with, an inert diluent, such as alumina or silica. I may use, for example, a granular mixture by weight of 70 parts barium oxide and 30 parts aluminum oxide which, after fusion and grinding, can be kept a long time without deterioration even in contact with air. A reducing agent, such as comminuted tungsten, molydenum, titanium, or tantalum, may be present -admixed with the oxide, but the metal of the dispenser shell or envelope which may consist of molybdenum or tungsten may constitute the reducing agent.

I have discoveredV that from such a dispenser a thermionically active material is produced when the dispenser is heated to a temperature higher than the temperature of the cathode, and that such material will diffuse through the pervious enclosure to the surface and will become transferred to the adjacent cathode surface making it active, that is, highly electron emissive when heated to an emitting temperature. For example,

when the reducing agent is molybdenum or' tungsten, I have obtained excellent results when the dispenser was heated to about 1100 to 1200 C., the cathode being heated to a temperature of 800 to 900 C. As cathode members have been found by experience to be activated -by the action of such a dispenser for long periods, and the vapor of such material to exert no eiIect on the ionization characteristics of the gas illling of the device, it appears that auch. material has aninappreciable vapor pressure. While the 4exact composition of this active material has not been definitely determined, I believe it to be a mixture of alkaline earth metal and oxide, plus some compound thereof with the dispenser metal. The

electron-emitting layer forms most readily on a molybdenum cathode member.

The heated cathode emits copiously as soon as a coating of the -order of molecular thickness has been transferred thereto from the dispenser. As this coating layer is continually renewed, it need never be any thicker than a monomolecular layer provided the temperatures of the dispenser y said members. It has been found that an accumulation of activating material on the cathode surface, up to about three milligrams per square centimeter of cathode surface, does not harm nor materially effect the electron emission.

While best results are obtained with clean, un

coated cathode surfaces, an initial coating may be used advantageously in some cases, for example, an oxide of alkaline earth metal or of the foundation metal, or a combination of such oxides.

The modified form of cathode and dispenser lassembly shown in Fig. 2 is provided with a dispenser 35 which is provided with a central wire 35 which may consist of tungsten or molybdenum. This Wire 35 is surrounded by an insulating tube 31 of refractory material, such as alumina or baryllia, which separates the dispensing charge of oxidic material from. the central supporting Wire 36. A similar wire 35 has been indicated in Fig. l, and may be used in a dispenser of the form shown in Figs. 1 and 2,A even without the insulating tube 31. While it is not an essential feature of my invention, I.prefei; to construct a dispenser with a core wire to be used either as a support or as part of the heating circuit for the dispenser. It may function as the main heating means for the dispenser. The structure of the cathode shown in Fig. 2 is similar to that shown in Fig. 1. However, the orifices 38 for the heat shield 6' are provided at the side instead of at the top.

In the modification shown in Fig. 3, the heaterdispenser I0 is wound helically on a supporting tube Il of refractory material, such as alumina or beryllia, which is also provided with a central supporting wire 42. A s shown in Fig. 3a, the heater-dispenser consists of an external sheath 43 of woven material providing pore-like crevices and may be constituted of tungsten or molybdenum wires. It is filled with a charge Il of granular oxides as already described above. Such oxides need not be associated with a reducing agent. The cathode ribbon members, such as shown at 45 in Figs. 1, 2,` and 3, may have provided on their surface a woven wire structure 46 as shown in Fig. 4. The wires 45 serve to increase the electron-emitting area of the cathode, and, in case an oxidic coating is to be used, to assist in holding such coating in place.

Various changes and modifications may be made in cathodes embodying my invention without departing from my invention. For example, the alkaline earth material may be introduced into the dispenser as a paste, or as preformed pellets. The dispenser may be heated by current passing through the core wire (such as 315, Fig. 2) and the sheathin series, these elements being connected at one'end by the plug 41. The core wire, Fig. 2, is supported by a plate 48, resting on the insulator 49.

Figs. 5 and 6 show a dispenser, illustrated diagrammatically, Vconsisting of a heater wire l wound about a preformed rod I of a composition capable of yielding an electron-enhancing material. This composition may consist of alkaline earth oxide and a reducing agent, with or without a stabilizing diluent, such as alumina. The dispenser is located within a cylindrical cathode5 2, having upon its inner surface a number of projecting ilns 53. The inner wall member of the cathode!! and also the fins 53 preferably consist of molybdenum or tungsten. An exterior heat shield Il is provided which may consist of multiple spaced cylinders as indicated, and which may be insulated from the cathode as by a coating of alumina. The supply conductors 5I and 58 are insulated from the metal parts by the insulators 51, il. As indicated, the upper end of the heater wire Il is connected to the cathode 52 by a transverse conductor Il.

As shown in Fig. 7, the dispenser may be electrically unconnected to and physically separated from the electron-emitting elements or ribbons. In the structure here shown, the emitting elements ll and I2 are supported by a plate 68 which is held between adjacent members of insulator 63. The heat shield 6I likewise is supported by the insulator 63. 'Ihe dispenser 85 is similar in structure to the dispenser II'of Fig. 2, the sheath and core being connected electrically by a conducting link 88, which as shown is electrically insulated from the electron-emitting elements 6I, 62. The dispenser may be heated by current traversing the core and sheath in series.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical discharge device comprising the combination of an envelope, an attenuated gas therein, an anode, a thermionic cathode con- 50 sisting of a plurality of strips of bare molybdenum which are radially arranged about a common center, an elongated dispenser of material for activating said cathode, said dispenser being 5 axially disposed with respect to said cathode strips and comprising a container of woven wire of a metal chosen from the group consisting of tungsten and molybdenum, a charge therein consisting of a mixture of oxide of barium oxide and alumina,means for heating said dispenser, and a shield surrounding said cathode and dispenser. 2. A. cathode structure comprising an activating assembly which includes a iinely perforated container enclosing alkaline earth material in vaporizable form together with a resistance heater in effective heat-exchanging relation with said-material, and one or more thin metal emitting elements positioned to receive both radiant heat and vaporized alkaline earth material from said as- 20 sembly, said elements being constituted of a metal which is capable of electronic activation by the addition of alkaline earth material and being maintained at a temperature of effective electron-emissivity by said radiant heat during normal use of the cathode.-

3. An electrode arrangement including a heat shield, one or more thin metallic electron-emitting members disposed within the heat shield, and an activating assembly comprising a iinely perforated container which encloses alkaline earth material in vaporizable form and a resistance heater in eilective heat-exchanging relation with the container, said assembly being positioned to supply both vaporized alkaline earth material and radiant heat to the said electron-emitting members.

4. An electrical discharge device comprising the combination of an envelope, an attenuated gas therein, an anode, a thermionic cathode comprising a plurality of thin metallic elements which are radially arranged about a common center, an elongated dispenser of material for activating said cathode, said dispenser being axially arranged with respect to said metallic elements and comprising a finely perforated metal container, a

charge therein comprising alkaline earth material, means for heating said dispenser and a shield surrounding said cathode and dispenser.

ALBERT W. HULL. 

